Field of the Invention
The invention relates to a plasma spraying apparatus which transfers plasma-arc to an electrically conductive wire to thereby generate plasma flame, melts the wire into droplets, and sprays the droplets onto a target, and further, to a method of forming a sprayed coating onto a target.
Description of the Related Art
FIG. 1 is a cross-sectional view of a conventional plasma spraying apparatus.
As illustrated in FIG. 1, the conventional plasma spraying apparatus 90 includes a first gas nozzle 91 defining a first gas path 91a, a second gas nozzle 92 disposed outside of the first gas nozzle 91 to define a second gas path 92a, a cathode 93 disposed substantially on central axes of both a nozzle opening 91b of the first gas nozzle 91 and a nozzle opening 92a of the second gas nozzle 92, a battery unit 94, and a wire guide hole 95 for introducing an electrically conductive wire W to be sprayed, into a vicinity of the nozzle opening 92a of the second gas nozzle 92.
The wire W is supplied obliquely of a central axis of the nozzle opening 92a and in front of the nozzle opening 92a through the wire guide hole 95. A first gas sprayed through the first gas path 91a is turned into plasma flame F by means of arc generated between the wire W indirectly electrically connected to an anode of the battery unit 94 through the second gas nozzle 92, and the cathode 93 electrically connected to a cathode of the battery unit 94. The thus generated plasma flame F melts the wire W into droplets D, and sprays the droplets D. The droplets D are further reduced in size and further accelerated by a second gas sprayed forwardly of the second gas nozzle 92 through the second gas path 92a, and sprayed onto a target T to thereby form a sprayed coating S on the target T.
In the conventional plasma spraying apparatus 90, the first gas generally comprises an inert gas such as a nitrogen gas and an argon gas, and the second gas generally comprises compressed air, a nitrogen gas or a carbon dioxide gas (for instance, see Japanese Patent Application Publication No. H9 (1997)-308970).
In actual operation of the conventional plasma spraying apparatus 90, the use of a nitrogen gas or a carbon dioxide gas as a second gas increases operation cost, and hence, there is generally used compressed air for reduction of the operation cost. In the plasma spraying apparatus 90, the compressed air used as a second gas surrounds the first gas having been turned into plasma, and thus, it is possible to thin the first gas flow, and further, accomplish a high-speed first gas.
However, since the droplets D derived from the molten wire W are further thinned by the second gas jet, and the droplets D are given a sufficient speed in the conventional plasma spraying apparatus 90, there are generated externally of the plasma flame F because metals to be contained in the sprayed coating S are rapidly restricted during being molten by the compressed air used as a second gas. This results in that the droplets D are oxidized at surfaces thereof, and thus, the sprayed coating S contains therein metal oxide.